Edit: Looks like you can opt-out of that "new look and feel" pretty easily under the appearance settings but still, whats with putting rounded corners everywhere?
Edit 2: "Explore the web with a softer, more friendly aesthetic featuring rounded corners […] Designed to complement your operating system, whether on Windows 11, MacOS, or Linux." The fuck does that mean? Windows 11 fair enough but most Linux distros don't look like that at all.
Use different user accounts. That provides you with very stronger isolation and separation of concerns, with the bonus that you won’t be exposed to their crap.
How about you just let them use whatever works for them?